The solution to the speed problem for the rides around here is simple: start earlier. (Most of our long rides don't have a mass start.)

I've also heard the "Rule of 3" and it makes sense to me: on a supported ride you can go 3 times as far as you can go on a "regular" daily ride.

Generally, the people who worry about whether they can do it most don't need to worry

What I would do is start with the 100km - pace yourself and see how it goes. Then think about the 200 and the 250. Of course, 200 km feels like ten times as long 'cause you've already done 100kmwhen you started! As somebody already said, though, at a certain point the seat's most important.